The present invention relates to a method or apparatus for scheduling for a dual engine printer and, more particularly, but not exclusively to a dual engine printer that prints multi-colored spreads in a series of separations wherein all of the separations are carried out using a single drum set.
Dual engine printers are printers that have dual printing engines or dual sets of drums for printing. Typically dual engine printers are used in double sided, or duplex, printing, such that the one side of the sheet is printed using one engine and then the sheet is passed on to the second engine for the printing on the second side.
Conventional dual engine color printers have a series of printing drums or drum sets, one set for each color. Thus a four color printer has four drum sets in each engine.
A color printer used by the present inventors uses a drum set that has a drum set for each printing engine, the set comprising an organic photoconductive (OPC) drum, otherwise known as a photo imaging plate (PIP) drum, a blanket drum, and an impression drum.
The above color printer uses a single drum set in each engine that prints multi-colored spreads in a series of separations. Thus each of the engines of the dual engine printer comprises a single set of the Photo imaging plate (PIP) drum, blanket and impression drums. The print medium, or sheet, approaches the impression drum and is wrapped thereon. In a series of rotations the sheet passes the nip or image transfer position several times, each time receiving an image from the blanket in a different color. Once the full range of colors has been printed the sheet is removed from the impression drum and inverted as necessary for the second engine where the same procedure is repeated for the second side.
Now it is quite typical in duplex color printing, that only one side of the sheet has a color image. The second side is monochrome. Thus in color magazines for example, it is quite common to have a color image on one side only, with text on the reverse side.
In such a case a conventional dual engine printer that may have separate impression drums for each color simply passes the sheets through the engine one by one in the usual way. For monochrome printing no image is transferred by the remaining color drums.
However where each engine comprises a single drum set that does multiple colors, the sheet is detained at the impression drum for the number of rotations equaling the number of colors. The system operates smoothly as long as both sides require the same number of colors, but color-one-side monochrome-one-side printing leads to an imbalance in the utilization of the drums since one engine has to wait until the other has finished, slowing down the throughput of the sheets.
Thus by example we consider a print job requiring numerous sheets to be printed with a 6-color image on the first side and a monochrome image on the second side. The first sheet reaches the first engine where the 6-color image is to be added. The sheet is detained for six separate rotations of the drum during which time the second engine does nothing. Then the sheet is passed on to the second engine and a new sheet is taken by the first engine. The second engine performs a single drum rotation for its monochrome printing. The second engine's drum performs its single monochrome rotation simultaneously with the first rotation of the first engine for its second spread, but the second engine still has to wait idly for five more rotations, whilst the first engine continues to print its second color image, before the second can receive the second sheet. Thus the second engine spends 83.3% of its time idling and the overall utilization of the dual engine printer is reduced. Printing takes six rotations per sheet with no saving due to the fact that one side requires fewer colors.
U.S. Pat. No. 5,710,635 discloses a general purpose scheduling algorithm for a dual engine printer. It is applicable to situations having double sided and color printing. However, it is not specifically applicable to the present situation nor does it suggest a solution to the above problem.
U.S. Pat. No. 6,259,884 teaches a complex system for duplex printing which allows for improved printing efficiency. However, the system taught suffers from undue complexity.
U.S. Pat. No. 5,568,246 describes a dual engine printer and a method for scheduling single sided printing thereon by sending alternate sheets to the different engines. The result is greater utilization of the dual engine printer, but there is no teaching of how the method could be modified for unbalanced dual sided printing.
There is thus a widely recognized need for, and it would be highly advantageous to have, a dual engine single drum set color printing system devoid of the above limitations.